scholarly journals KORDONKA TERRANE — A FRAGMENT OF THE EARLY PALEOZOIC ACTIVE MARGIN

2021 ◽  
pp. 61-72
Author(s):  
V.V. Golozubov ◽  
◽  
A.I. Malinovsky ◽  
A.A. Chashchin ◽  
A.T. Kandaurov ◽  
...  
Keyword(s):  
Late Phase ◽  
Tectonic Block ◽  
Late Permian ◽  
Early Paleozoic ◽  
Primorsky Krai ◽  
Lower Silurian ◽  
Terrigenous Rocks ◽  
Central Asian ◽  
Geochemical Study ◽  
Middle Paleozoic

The Kordonka Formation (lower Silurian) forms a lens-shaped tectonic block up to four km wide, extending southward from the Pogranichny village area (Primorsky Krai) for about 30 km. The formation is composed of argillaceous slate, volcanic-sedimentary rocks with horizons of sandstones, siltstones, conglomerates, and cherts, and also contains flows of basalts and andesite-basalts. The results of lithological-geochemical study of the terrigenous rocks of the formation indicate that the Kordonka Formation was accreted in the outer pre-oceanic part of the paleo-island arc system. The eastern part of the formation is intruded and partially assimilated granitoids of Middle Paleozoic and Late Permian age. The early phase of the granitoids was dated by U-Pb zircone 373.5±1.7 and 383.5±1.6 Ma (Late Devonian). The late phase is characterized by 261.2±1.2, 257.4±0.93 and 261.5±1.6 Ma (Wuchiapingian and Capitanian stages of the Middle and Late Permian). The Devonian granitods first identified in this area may be evidence of the Kordonka block accession to the Bureya-Jiamusi-Khanka orogenic belt consolidated during the Ordovician. The Kordonka terrane thus marks an independent Early-Middle Paleozoic episode in the formation and closure of the Central Asian paleo-ocean.

Challenging the dipolar paradigm for Proterozoic Earth

2022 ◽  
Author(s):  
James W. Sears
Keyword(s):  
Magnetic Field ◽  
Geomagnetic Field ◽  
Inner Core ◽  
Ultraviolet B ◽  
Late Permian ◽  
Early Paleozoic ◽  
Thin Spherical Shell ◽  
Late Neoproterozoic ◽  
Geocentric Axial Dipole ◽  
Drift Path

ABSTRACT A robust, geology-based Proterozoic continental assembly places the northern and eastern margins of the Siberian craton against the southwestern margins of Laurentia in a tight, spoon-in-spoon conjugate fit. The proposed assembly began to break apart in late Neoproterozoic and early Paleozoic time. Siberia then drifted clockwise along the Laurussian margin on coast-parallel transforms until suturing with Europe in late Permian time. The proposed drift path is permitted by a geocentric axial dipole (GAD) magnetic field from Silurian to Permian time. However, the Proterozoic reconstruction itself is not permitted by GAD. Rather, site-mean paleomagnetic data plot ted on the reconstruction suggest a multipolar Proterozoic dynamo dominated by a quadrupole. The field may have resembled that of present-day Neptune, where, in the absence of a large solid inner core, a quadrupolar magnetic field may be generated within a thin spherical shell near the core-mantle boundary. The quadrupole may have dominated Earth’s geomagnetic field until early Paleozoic time, when the field became erratic and transitioned to a dipole, which overwhelmed the weaker quadrupole. The dipole then established a strong magnetosphere, effectively shielding Earth from ultraviolet-B (UV-B) radiation and making the planet habitable for Cambrian fauna.

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